A Mild Route To P₂

Although the diatomic form of nitrogen is readily available from the air and is exceedingly stable, the diatomic form of phosphorus???the element located just below nitrogen in the periodic table???is rather hard to come by. This dichotomy has stymied chemists who would like to be able to use diphosphorus (P₂) to make new kinds of phosphorus-containing molecules.

Now there may be reason to celebrate: Chemistry professor Christopher C. (Kit) Cummins and coworkers at MIT have developed a mild procedure for generating P₂ or its synthetic equivalent in solution (Science 2006, 313, 1276). The method promises to greatly expand the range of compounds containing the P₂ moiety, such as phosphine ligands for new catalysts.

"We've added a new kind of reactive intermediate to the synthetic chemist's toolbox," Cummins tells C&EN.

The stable molecular form of phosphorus is tetrahedral P₄ (white phosphorus). P₄ can be broken down into reactive P₂, but that requires temperatures in excess of 1,100 K, which is not realistic for organic synthesis.

The MIT advance involves the straightforward synthesis of a niobium complex containing a ligand with the P=P=NR linkage. Joshua S. Figueroa, the graduate student who first synthesized this complex, informally dubbed it "the eliminator." That's because the compound, when heated to 65 °C in solution, extrudes P₂ as a reactive intermediate. When the extrusion is carried out in 1,3-cyclohexadiene, two molecules of the diene sequentially add to the two bonds of P₂, forming a tetracyclic compound containing a P-P bond. This reaction has not been reported previously, the researchers note.

When the eliminator complex is heated in solution in the absence of a molecule that can trap P₂, the extrusion occurs as before, but the MIT chemists haven't been able to figure out what happens to the P₂. It might be polymerizing or adsorbing to the reactor walls, Cummins speculates.

In an effort to stabilize P₂ in solution after its extrusion, Cummins and coworkers Nicholas A. Piro and Jessica T. McKellar prepared an analog of the eliminator complex in which the ligand's terminal phosphorus atom is coordinated to a tungsten pentacarbonyl group, W(CO)₅. They found that this modified complex eliminates a (P₂)W(CO)₅ intermediate at room temperature, and this intermediate is trapped by 1,3-cyclohexadiene and other dienes to yield the W(CO)₅ adduct of the expected organodiphosphorus product.

Experimental data suggest that (P₂)W(CO)₅ has a longer lifetime than P₂, although it's still transient. Cummins' group is looking for P₂ species having even longer lifetimes.

"That's the neat thing about this work: It opens up a huge number of avenues for possible research," Cummins says.

Other chemists agree. James J. Kiddle of Western Michigan University, Kalamazoo, points out that the ability to generate P₂ will enable fundamental studies to explore molecular structure, bonding, and reactivity among the group 15 elements.

For phosphorus chemists, Cummins' paper is "the highlight of the year," comments Guy Bertrand of the University of California, Riverside. "It will stimulate other groups to work on P₂."